1. Field of the Invention
This invention relates generally to the recording and reproducing or other transmission of information signals, such as, video signals, and more particularly is directed to the processing of such video or other information signals so as to eliminate or at least reduce interfering or noise signals resulting from the recording and reproducing or other transmission thereof.
2. Description of the Prior Art
It is well known to record video signals on a magnetic tape or other record medium by scanning successive parallel tracks on the record medium with one or more transducers energized by the video signals. In effecting such recording of video signals, it has been the usual practice to provide guard bands or unrecorded gaps between the successive parallel tracks so that, when a transducer scans one of the tracks for reproducing the signals recorded therein, such transducer will not also reproduce cross-talk, that is, signals recorded in the adjacent tracks. However, the provision of guard bands between the successive parallel tracks reduces the recording density, that is, the amount of signal information recorded on a unit area of the record medium, and thus does not permit the efficient utilization of the record medium for the recording of video signals.
One effort made to minimize cross-talk while permitting an increase in the recording density has been to use two transducers having air gaps with different azimuth angles for recording and reproducing signals in the next adjacent or alternate tracks, respectively. This is relatively easy to do because apparatus for magnetically recording and/or reproducing video signals usually includes a rotary guide drum provided with two alternately operative transducers or heads which can have air gaps with different azimuth angles. The tape is wrapped helically about a portion of the perimeter of the drum and is moved longitudinally while the transducers or heads are rotated, thus causing the heads alternately to scan respective tracks for recording or reproducing signals therein. Each transducer or head, in the recording operation of the apparatus, effects magnetization of magnetic domains in the magnetic coating on the tape in what would appear to be, if such domains were visible, a series of parallel lines or stripes each having a length as great as the width of the track, and each having an orientation that corresponds to the azimuth angle of the gap of the respective transducer or head. In the reproducing or playback operation of the apparatus, each track is scanned by the transducer or head having its gap aligned with the parallel, but fictitious, lines of that track, from which it follows that the gap of the transducer or head scanning a track for reproducing the video signals recorded therein extends at an angle to the mentioned fictitious lines of the tracks next adjacent to the track being scanned. By reason of the foregoing, if a transducer or head, in scanning a track for reproducing the video signals recorded therein, overlaps an adjacent track or otherwise reproduces signals recorded in the latter, the well-known azimuth loss would result in attenuation of the signals reproduced from the adjacent track.
It is also well known that, in recording video signals as described above, it is advantageous to record at least a portion of the video signals as a frequency modulation on a carrier having a relatively high frequency. Since the previously mentioned azimuth loss is generally proportional to the frequency of the signals, such azimuth loss is relatively effective to decrease or eliminate the cross-talk from adjacent tracks in respect to the frequency modulated portion of the video signals recorded in the tracks. Thus, insofar as the frequency modulated portion of the recorded video signals is concerned, it is possible to record the video signals in abutting or even partially overlapping successive parallel tracks, that is, without providing guard bands between the adjacent tracks on the record medium.
Although the elimination of the guard bands substantially increases the recording density, any further increase in the recording density, and hence in the amount of video signal information or playing time that can be accommodated on a given length of the magnetic tape, can be achieved only by reducing the width of the successive parallel tracks. However, the degree of azimuth loss is inversely proportional to the width of the tracks. Therefore, if the width of the tracks is reduced to less than a predetermined value, the interference due to cross-talk between video signals reproduced from adjacent tracks is not eliminated or sufficiently minimized by the use of transducers or heads having different azimuth angles. It will be apparent that, if the magnitude of the cross-talk signals being reproduced by a transducer or head from tracks adjacent the track being scanned thereby is not sufficiently small in relation to the magnitude of the video signals reproduced by the head from the scanned track, an interference or beat signal with a frequency different from that of the video signals reproduced from the scanned track and the cross-talk signals will appear as a beat or moire pattern on the image or picture displayed by the cathode ray tube.
Further, it is the usual practice to record video signals with so-called H-alignment so as to avoid interference or cross-talk from the horizontal synchronizing and blanking signals included in the video signals recorded in the successive parallel tracks. In recording video signals with H-alignment, the ends of the margins between the successive areas in which line intervals are recorded in each track are aligned, in the direction transverse to the lengths of the tracks, with the adjacent ends of the margins between the successive areas in which line intervals are recorded in the next adjacent tracks. However, it is not possible to provide an apparatus in which the video signals are recorded with H-alignment when the tape is advanced at two or more different speeds. For example, if the video signals are recorded with H-alignment when the tape is longitudinally driven or transported at a predetermined speed, the signals will not be recorded with H-alignment when the tape is transported at one-half such predetermined speed. In that case, the level of the interfering or noise signal due to cross-talk will become high, for example, when a head or transducer, in reproducing picture information from a track being scanned, simultaneously reproduces a horizontal synchronizing signal as cross-talk from an adjacent track, so that there is a relatively great frequency difference between the cross-talk and the video signal picture information being reproduced from the scanned track. Thus, once again, a beat or moire pattern will appear on the image or picture displayed by the cathode ray tube.
When recording color video signals which include a luminance component and a chrominance component, it is known to separate such components and then to frequency modulate the luminance component on a carrier having a relatively high frequency, while the chrominance component is frequency converted so as to have its frequency band shifted below the frequency band of the frequency modulated luminance component, whereupon the frequency modulated luminance component and the frequency converted chrominance component are combined to provide composite video signals which are recorded in the successive parallel tracks. However, since the azimuth loss is generally proportional to the frequency of the signals, as previously mentioned, interference due to cross-talk from the low frequency or frequency converted chrominance component is not reduced to the same degree by the use of transducers having different azimuth angles as cross-talk from the high frequency or frequency modulated luminance component. Thus, when recording color video signals, it has been proposed, for example, as disclosed in detail in U.S. Pat. No. 4,007,482 and No. 4,007,484, issued Feb. 8, 1977, and each having a common assignee herewith, to reduce or eliminate interference due to cross-talk between low frequency signals recorded in adjacent tracks by recording the chrominance component with different first and second carriers in such adjacent tracks, respectively. The first and second carriers modulated by the chrominance component for recording in adjacent tracks, respectively, may be distinguished from each other by their respective polarity characteristics so that, when a head scans a particular track for reproducing the video signals recorded therein, the chrominance component of cross-talk signals from the tracks next adjacent to the scanned track can be conveniently suppressed or eliminated by reason of the different polarity characteristics of the carriers with which the chrominance component was recorded in the scanned track and in the tracks adjacent thereto, respectively. Although the foregoing scheme effectively eliminates interference due to cross-talk in respect to the chrominance component while permitting a high recording density to be achieved by eliminating guard bands between the tracks and reducing the width of the latter, the previously mentioned limitations on the reduction of the width of the tracks still apply in respect to the frequency modulated luminance component of the recorded color video signals in that, as to such frequency modulated luminance component, the elimination of interference or cross-talk still depends upon the use of transducers or heads having different azimuth angles for recording the video signals in the next adjacent tracks.
Moreover, in the recording operation of existing apparatus for recording and reproducing video signals, at least a high frequency portion of the video signals, that is, the luminance component, is pre-emphasized in a pre-emphasis circuit prior to the frequency modulation thereof, and, in the reproducing or playback operation of such apparatus, the reproduced frequency modulated luminance component is demodulated and then de-emphasized in a de-emphasis circuit which has a characteristic complementary to that of the pre-emphasis circuit for reducing the so-called FM noise signal from the demodulated luminance component. However, the level of the FM noise signal increases with the frequency of the carrier of the frequency modulated luminance component so that, if a high carrier frequency is employed, as previously mentioned, the FM noise signal has a correspondingly high level. Further, the previously mentioned interfering signal due to cross-talk cannot be removed or eliminated by the usual pre-emphasis and de-emphasis circuits because the level of the interfering signal is larger than that of the oridinary FM noise signal and its frequency characteristic is different therefrom. If the amount of emphasis provided by the pre-emphasis circuit is merely increased for the purpose of eliminating the high level FM noise signal and minimizing the interfering signal due to cross-talk upon a corresponding or complementary de-emphasis, relatively large overshoots are produced at those portions of the video signal where the level of the latter is abruptly changed. If the emphasized video signal with such large overshoots is employed as the modulating signal for the frequency modulation, the frequency shifts corresponding to such large overshoots are too great to be included in the transmission band of the apparatus, that is, the band of frequencies that can be magnetically recorded and reproduced, so that the recording and reproducing of the video signal cannot be effected with a linear characteristic. In order to bring the frequency modulated video signal, as recorded and reproduced, within the transmission band, the existing recording and reproducing apparatus is provided with a white and dark clip circuit which limits the pre-emphasized video signal at predetermined high and low levels and thus clips some of the overshoots. In the case of such limiting of the pre-emphasized video signal, however, even if the video signal is processed in a de-emphasis circuit after demodulation during the reproducing operation, it is not possible to obtain precisely the original video signal owing to the distortion of waveforms at the limited or clipped overshoot portions.